A nitride semiconductor including nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting device because its bandgap is sufficiently wide. Among other things, gallium nitride-based compound semiconductors (which will be referred to herein as “GaN-based semiconductors” and which are represented by the formula AlxGayInzN (where 0≦x, y, z≦1 and x+y+z=1)) have been researched and developed particularly extensively. As a result, blue-ray-emitting light-emitting diodes (LEDs), green-ray-emitting LEDs and semiconductor laser diodes made of GaN-based semiconductors have already been used in actual products.
A GaN-based semiconductor has a wurtzite crystal structure. FIG. 1 schematically illustrates a unit cell of GaN. In an AlxGayInzN (where 0≦x, y, z≦1 and x+y+z=1) semiconductor crystal, some of the Ga atoms shown in FIG. 1 may be replaced with Al and/or In atoms.
FIG. 2 shows four fundamental vectors a1, a2, a3 and c, which are generally used to represent planes of a wurtzite crystal structure with four indices (i.e., hexagonal indices). The fundamental vector c runs in the [0001] direction, which is called a “c axis”. A plane that intersects with the c axis at right angles is called either a “c plane” or a “(0001) plane”. It should be noted that the “c axis” and the “c plane” are sometimes referred to as “C axis” and “C plane”. In fabricating a semiconductor device using GaN-based semiconductors, a c-plane substrate, i.e., a substrate of which the principal surface is a (0001) plane, is used as a substrate on which GaN semiconductor crystals will be grown. FIG. 3(a) schematically illustrates the crystal structure of a nitride-based semiconductor, of which the principal surface is a c plane, as viewed on a cross section that intersects with the principal surface of the substrate at right angles. In a c plane, however, there is a slight shift in the c-axis direction between a Ga atom layer and a nitrogen atom layer, thus producing electrical polarization there. That is why the c plane is also called a “polar plane”. As a result of the electrical polarization, a piezoelectric field is generated in the InGaN quantum well of the active layer in the c-axis direction. Once such a piezoelectric field has been generated in the active layer, some positional deviation occurs in the distributions of electrons and holes in the active layer due to the quantum confinement Stark effect of carriers. Consequently, the internal quantum efficiency decreases, thus increasing the threshold current in a semiconductor laser diode and increasing the power dissipation and decreasing the luminous efficacy in an LED. Meanwhile, as the density of injected carriers increases, the piezoelectric field is screened, thus varying the emission wavelength, too.
Thus, to overcome these problems, it has been proposed that a substrate, of which the principal surface is a non-polar plane such as a (10−10) plane that is perpendicular to the [10−10] direction and that is called an “m plane”, be used. As shown in FIG. 2, the m plane is parallel to the c axis (i.e., the fundamental vector c) and intersects with the c plane at right angles. FIG. 3(b) schematically illustrates the crystal structure of a nitride-based semiconductor, of which the principal surface is an m plane, as viewed on a cross section that intersects with the principal surface of the substrate at right angles. On the m plane, Ga atoms and nitrogen atoms are on the same atomic plane. For that reason, no electrical polarization will be produced perpendicularly to the m plane. That is why if a semiconductor multilayer structure is formed perpendicularly to the m plane, no piezoelectric field will be generated in the active layer, thus overcoming the problems described above. In this case, the “m plane” is a generic term that collectively refers to a family of planes including (10−10), (−1010), (1−100), (−1100), (01−10) and (0−110) planes.
Also, as used herein, the “X-plane growth” means epitaxial growth that is produced perpendicularly to the X plane (where X=c or m) of a hexagonal wurtzite structure. As for the X-plane growth, the X plane will be sometimes referred to herein as a “growing plane”. Furthermore, a layer of semiconductor crystals that have been formed as a result of the X-plane growth will be sometimes referred to herein as an “X-plane semiconductor layer”.